Transition radiation (TR) occurs when a relativistic charged particle transitions across a boundary between two materials with different relative permittivities, also known as dielectric constants. The TR broadband radiation phenomenon is well understood and investigated and is utilized in a variety of high energy physics, nuclear physics, and satellite detectors and in a variety of beam line monitors for particle beam accelerators. TR has been minimally utilized for photon sources in detectors of relativistic charged particles and it has been suggested that this might be possible to make more intense TR sources of photons. TR intensity is roughly proportional to the relativistic particle's energy, E, and the opening half angle of the TR photon radiation along the particle's path is roughly the Lorentz factor, γ=E/mc2.
TR photon sources are not widely developed, as the technology to date does not produce sufficient photon intensity to be of use in most situations.
Materials utilized for TR radiators have typically been thin silicon wafers or metal foils, including multiple layers of foils, or layers of randomly oriented foams or fibers of polyethylene, polypropylene, or similar materials. Depending on the system, the foils, foams, or fibers may be in vacuum, gas, or other material that allows the TR photons of interest to pass through the material. The design goals of typical TR systems include but are not limited to optimizing the number of useable photons, optimizing the distribution and positions of the TR photons, minimizing the self-absorption of the TR photons by the TR material, and maximizing the number of TR surfaces in the overall TR system.
Accelerator systems associated with relativistic charged particle beams, energy recovering linacs, multiple beam accumulation and beam rastering are known by the international accelerator community, but without an effective transition radiation material have limited use in generating TR beams of photons beyond relativistic charged particle detection.